Carbonized metal and method of making it



1955 J. B. DIFFENDERFER 2,719,355

CARBONIZED METAL. AND METHOD OF MAKING IT Filed June 28, 1947 2 Sheets-Sheet l INVENTOR John B. Difiezzderfer 1955 J. a. DIFFENDERFER 2,719,355

CARBONIZED METAL AND METHOD OF MAKING IT Filed June 28, 1947 2 Sheets-Sheet 2 INVENTOR.

ATTORNEYS.

United States Patent CARBONIZED METAL AND METHOD or MAKING IT John B. Dilfenderfer, Catlin Township, Chemung County,

N. Y., assignor to Radio Corporation of America, a corporation of Delaware Continuation of abandoned application Serial No. 757,752, June 28, 1947. This application October 3, 1952, Serial No. 312,945

3 Claims. (Cl. 29-195) My invention relates to carbon coated metal for use where good thermal emittance or heat radiating power is desirable as, for example, in evacuated devices, such as electric discharge devices where metal parts or elements radiate heat in vacuum during operation of the device. In electronic discharge devices like radio tubes such carbonized metal is particularly useful for tube elements, such as carbonized anodes' or plates, for heat radiators attached to tube elements, such as grids, and for similar uses. Myinvention further relates to carbonized metal or plate material for electrodes for electric discharge devices, to methods of making such material, and 'to improved carbonized electrodes, such as plates or anodes, for use in thermionic discharge devices.'

This is a continuation of my co-pending application Serial No. 757,752, filed June 28, 1947, for Carbonized Metal and Method of Making it, now abandoned, and assigned to the same assignee as said co-pending application.

In electronic discharge devices the metal parts, other than the electron emissive elements, are generally kept as cool as feasible. To this end, in thermionic discharge devices, such as radio tubes, these other or cold electrodes such as grids, plates, and other electrodes which receive or are subjected to the electron current during operation are blackened to increase their thermal emittance or heat radiating power, usually by a coating of some form of carbon. For reasons of economy in manufacture on a large scale, long strips of sheet electrode metal are usually carbonized by a continuous process and the electrodes then formed from the carbonized strip. Nickel has been used extensively as the metal for the electrodes. The ferrous metals, such as mild steel, iron, or iron alloys, are much cheaper than nickel and could be used if satisfactorily carbonized, but they could not be carbonized with satisfactory results. The conventional gas carbonizing method in which the metal is heated in a hydrocarbon atmosphere to a temperature at which the hydrocarbon. decomposes and deposits carbon on the metal is not satisfactory when applied to ferrous metals. The temperature necessary to obtain from the hydrocarbon atmosphere the right kind of carbon is so high that there is some carburization or case hardening of the ferrous metal, which is objectionable for several practical reasons. Gas carbonizing hardenssteel, even when the steel has a thin coat of nickel, and impairs the strength of the metal. As the electrodes must be made with great precision, die forming is difficult unless the carbonized strip is quite ductile and practically non-re- 2 ,719,355 Patented Oct. 4, 1955 other methods have been used to roughen the surface of the metal in eflorts to improvethe adhesion of the carbon coating. Experience has shown that adhesion of the carbon coating to the metal and the thermal emittance of the carbon coated electrode is a function of and is dependent on the texture of the metal surface rather than on its roughness.

The principal object of my invention is to provide metal, such as plate or electrode material, with a surface of novel texture to which a coating, such as carbon, adheres more firmly than to the surfaces obtained by conventional methods, and which when coated with carbon, especially carbon wholly or partly in the form of colloidal graphite, has a thermal emittance or heat radiating power which is very considerably greater than that of the best commercially used carbonized material of the conventional kind and closely approaches that of a black body.

Another object of the invention is to provide on an annealed ferrous metal such as mild steel a firmly adherent carbon coating so applied that the carbon does not affect or modify the properties of the metal in any way.

A further object of my invention is to provide a satisfactorily carbonized and commercially useful electrode of ferrous or other metal which may, if desired, be made to have a thermal emittance greater than 90% of that of a black body.

Another object of my invention is to provide electrode stock or material comprising a strip or sheet of ferrous or other metal which is practically dead soft, is easily shaped in dies into electrodes of precise dimensions, has a comparatively soft surface compressible in the forming dies, and preferably has a coat of carbon paint which is so firmly adherent that it will withstand the working of the material in the electrode forming dies.

Another object of the invention is to provide a simple, rapid and continuous process for treating a steel or similar metal strip to form onits surface a spongy porous layer or matrix of novel texture to which a coating, such as carbon, adheres very firmly and with which a carbon coating forms carbonized metal having thermal emissivity very close to that of a black body.

In accordance with my invention a base of sheet metal, preferably, but not necessarily, a ferrous metal such as mild steel, although some other metal, such as nickel, may be used, is surfaced by bonding to one or both of its surfaces a very firmly adherent porous layer,

or matrix bonded to the base and composed of fine metallic particles sintered to one another to leave voids forming fine pores. In the preferred embodiment the metallic particles are metal sponges which have microscopic silient. Any carburization of a ferrous metal strip depores and crevices, and which are sintered to one another and to the metal base to form a cohesive mass molecularly bonded to the base and having a novel texture and porosity or sponginess because of the porosity due to the minute pores or voids between the particles sintered to the base and to one another and also to the microscopic porosity of the individual particles or metal sponges. The matrix may be composed of particles which are nearly all about the same size, or it may be composed of a mixture of particles of different sizes. The surface of the matrix of thesurfaced base is usually rough and irregular, with a multitude of minute pits and depressions, and also of microscopic pits on the surface of the porous sponges or particles of metal. The layer or matrix of small sintered porous particles or sponges of metal has a novel texture and structure and is preferred because it has been found to hold a coating, such as carbon, more firmly than the porous matrix composed of fine solid particles of metal sintered to one another and bonded to the metal base. i

It The fine metal particles of the matrix are best obtained by the controlled reduction in place on the base of fine particles of oxide of the metal constituting the particles. For example, to? obtain the preferred. form. of matrix. composed of porous, particles, finely ground; metallic. 0.x-

ide, preferably nickel oxide. is. uniformly distributed.

over and in contact with the surface of: the. base as a. layer of powdered oxide with the grains. of the powder. in contactwith. one another and. held. on. the. foundation base by a binder which disappears when. moderately heated. The powderedoxide is reduced to metallicnickel by. heating the metal stripwiththe layerof nickeloxide. on; it in a. reducing atmosphere, such as hydrogen, pref-- erably at about 800 C. to ll C..until,the grains of nickel oxide; are reduced without fusion to, minute; nickel.

spongeswhich: have microscopic pores; andcrevices The temperature at which thenickel oxide is reduced. is: high faced with my novel matrix constitutes; a: foundation. which, when: properly carbonized, producesa-carbon-ized. electrode superior; to the; conventional; kind.- A coating.

such as carbon preferably consisting in large, part: of colloidal. graphite, applied. to; and. covering this; matrix isfound to-be.exceptionallyadherent... The. adhesion of. the carbon particles to the matrix: is; greater: than: their:

cohesion; to one another-,, hencetthe. carbon coating: cannot be. removed as; sheets.- orfiakes-,. but; only. as: carbon powder, the particles of.- which adhere: very. strongly. to

the foundation, Such a carbonized electrode; retains the.

carbon coating unusually well and has a. thermal: emit-- tance.or: heat radiating-powerr-which. may. be as .high: as 98%, of that. of: ablack body..

The. foundation' strip is. preferably. made by. a. CDntiH'ruous processin-v which.the'metal2basestrip,- coatedswith: a layer of powdered;- nickel. oxide,- is; passed: through a.

hydrogen furnacein: which the oxide isreduced, preferably at such a temperature and under: suchconditionsthat. the reduced metallic nickel forms porousparticles or nickel'sponges sinteredto one another-and bonded to the base, thus producing a. foundation consisting of a sheet'of metal surfaced with a layeror-matrix of-thedesired texture and structure.

The carbon may be applied: to the foundation in.various: ways, preferably asza kind'of carbon paint consisting of very. finely. divided graphite, lampblack: or other:

form of carbon suspended in a thin binder; The carbon paint may beapplied in. any. convenient way, such. as spraying, brushing, or. dipping, but preferably. byv a con-. tinuous immersion or drag coating process. in whichthefoundationwith the novelporous matrixisin the form. of astrip which is passedthrough athinsuspension of:

colloidal graphite in a vehicle suchas alcohol, to whichmay be added some; nitrocellulose or. similar binder.

Somevery finely. divided lampblack may be; addedif a very blackcoatingis desired.- Thesuspcnsionis of such low viscosity as tobe quickly-- absorbedzby the porous matrix, such asink. is absorbed by blotting paper, and this impregnatesthe matrixwith very finely divided carhem, and also cover the surface of the. matrix with a coat of the carbon paint. ing on it athin coat of the .carbonor-graphite paint with a surface which usually has in general muchthe same contour asthe rough and irregular surface of the, matrix. When the vehicle of the carbon paint is.driven.off there is left on the foundation a carboncoatingwhich: is practically bonded to the metal.

The foundation .is dried, leav-- My invention will be better understood from the following description in connection with the accompanying drawing in which merely for purposes of illustration there is shown one embodiment of my invention in a carbonized element, such asan anode or plate, of a thermionic tube, and my preferred method of making the anode or plate material. from which the finished anode or plate is made, and in which:

Fig. 1 illustrates the surface of the. porous matrix. as seen in a photomicrograph with a: magnification of 1000 diameters;

Fig. 2 illustrates the structure of the matrix as seen in. a photomicrograph with a. magnification. of 1000 diameters in cross-section;

Fig. 3 illustrates on a greatly enlarged scale the surface of the matrix. composed. of metal; sponges sintered to one another to leave voids and minute pores;

Fig. 4- illustrates on a. greatly enlarged scale a crosssection of the; matrixstructure illustrated-in Fig. 3;.

Fig, 5 illustrates. a. portion. of the. base coated. with: nickel oxide. as seen in crosssection in aphotomicro-- ph;

Fig. 6 illustratesinicrossrsection the base with-a matrixformed from the nickel oxide coatingofFig. 5 and seenas ina photomicrographtupon whichisshown merely for il-- lustrationv thecarbon coating. of the finished. element;

Fig. 7' is a view of a. carbonized plate part having two deep. drawn projections and four right angle; bends; and made successfully on a large scalein accordance with my invention; 7

Fig. 8 is a fragmentary cross-section of the. stripas itleaves the apparatus with acoat ofcarbon paintzon it;.

Fig. 9 shows in part schematically oneexample: of: the. apparatus suitable for practicing'my continuous: process, and Fig. 1O adetail of the device.

One specific embodiment of my invention is inta; car. bonized plate or'anode, having, as indicatedinthe drawing, a metal foundationcomprising a: non-porous. metal base.10,-,preferably a;sheet.of ferrous metal suchase cold rolledor mild-steel about: 5 mils thick,.surfaced. with. a; porous, metallic matrix 11- consisting predominantly of nickel: sponges or porous particles of nickel. llhaving. microscopic pores 13. and. firmly. andmolecularly bonded: tothe 132186710. and-also. sintered to one.- anothen. Photor micrographs show that; the. pores. 13. are: usually of an". average.- size: off about aztenth of. atmicron'. The. nickel.

sponges; or porous. particles; of nickel; retainL in: general;

their; individuality, and: are. sintered to;one;another,'.with2 voids 141between1them formingzminute. capillary. pores.l5? ma y; f whichlextend; through-the; matrix to the.basei10..; These pores 15.- are; in general many. times the. size; ofatlie: microscopic; pores; of thenickel sponges; and may often: be: about the sizeaof'. thenickel. particles. Phave-z foundithattIi may apply to; the matrixiaacarbonzcoating. preferablyxbut notznecessarily, sozthinthat' the outersurzfaceisqin general of: the: same contourasxthe interface' between the" matrix and the carbon 4 coatingand therefore of thecsame generalcontour as thezsurface. of therporous matrix. 11; andi achieve acarbonized: electrode having;- thermalcrnittanceor. heatrradiating-power: of: from%: to; 98%.. thattof atblack :body; with: the: carbon coating :of:

such: a, character; andso firmly adherentthat there is .prac-.-.

commercial practice by using surfaces roughened iH-1COflT- ventional ways.- The; adherence of-carbon .tometal iiS not solely dependent on thefldegreeor extent ofrroughness-cft themetal.surface. Thecharacterof the. roughness and. the-texture of .the. surface. are much more important... Me; chanical mughening ,Ofa :metal surface .by asinglesand.

blasting or steel blasting deteriorates the metal and decreases its ductility by cold working and produces a surface only about one-half as rough as the surface of the matrix 11, and of a texture, which does not bind the car ban to the metal nearly as well as the porous matrix 11. Blasting increases die wear because fine particles of the blasting material often adhere to the electrode metal. Repeated sand or steel blasting to increase the roughness of the surface deteriorates the metal still more, does not improve the texture for holding carbon, and is not feasible on account of cost and other objections. An oxidized strip of nickel or of nickel plated steel has only a very slightly roughened surface, which does not have the right texture to hold carbon as firmly as the matrix of my invention. Repeated oxidization and reduction of the nickel increases to some extent the roughness of the surface, but does not improve the texture for holding carbon, embrittles the nickel, and is not commercially feasible, largely on account of cost.

The matrix 11 of microscopically porous nickel sponges is most conveniently formed by applying to the base 10 a non-compacted layer of powdered nickel oxide in a binder, as indicated in Fig. 5, and reducing the nickel oxideunder such conditions and at such temperature that the oxide is reduced without fusion to form the nickel spon es in place on the base and simultaneously molecularly bond them to the base and sinter them to one another with minute voids between them. The layer of nickeloxide may be formed on the base in various ways, preferably by means of a slurry or suspension of the powdered oxide in a binder and applied in various ways, as by brushing, spraying, or by immersing of the base in the suspension. A dryspray results in a matrix with a very rough surface, and a wet spray produces a smoother surface. The preferred way of applying the oxide is by the immersion or drag coating method, in which the base is coated bypassing it through a bath of the suspension and then drying the coating. Commercial nickel oxide suitable for .use in practicing my invention may be obtained on the market as a mixture of nickelous and'nickelic oxides,.NiO and NizOa, and is believed to be prepared from a natural ore by refining to eliminate sulphur and other impurities. The oxide is preferably ground by ball milling to a powder so fine that most of the grains are about 2 microns in diameter and less than 5% of the grains are larger than 5 microns, although a minor part of the powder may in some cases consist of much larger grains. A typical batch of nickel oxide powdered by ball-milling consisted of about 80% to 85% of grains or particles about 2 microns in diameter, 5% or more of particles between 2 microns and 5 microns, and the remainder of particles ranging in diameter from 5 or 6 microns up to 16 microns. These nickel oxide grains or particles 16 reduce to nickel sponges which usually are about the same size as the nickeloxide particles from which they are formed. These sintered sponges produce a matrix with a very uniform fine grained surface, best shown inFig. 1, light grayish in color and about twice as rough as a single sand blasted nickel surface. It is obvious that a matrix with a rougher surface may be obtained by using larger grains of oxide. For immersion or drag coating the base with nickel oxide good results have been obtained with a slurry of the powdered nickel oxide suspended in an" evanescent binder consisting of highly volatile butylor amyl acetate and of some nitrocellulose. Suitable proportions are about 1500 grams of the nickel oxide powder to about 700 grams of amyl acetate and 35 grams of 40 second nitrocellulose. Some methanol, such as 250 grams, may be added to obtain the proper viscosity of the binder. This slurry coats the base evenly and when dried, leaves a suitable coat or layer of nickel oxide adhering to the base. I have found that-good results are obtained when this binder has a viscosity of about 30 to 35 C. P; S. at 23 C., or with a 6 standard pipette of to seconds for 100 cc. of binder at 23 C.

The nickel oxide is preferably reduced by heating the oxide coated base, best shown in Fig. 5, in a reducing atmosphere, preferably hydrogen, at temperatures and for times ranging from about 800 C. for about ten minutes to 1100 C. for about one-half minute, which will ordinarily reduce the particles 16 of nickel oxide to nickel sponges 12. A desirable and convenient procedure is to heat to about 900 C. for about one minute. The preferred amount of reduced nickel constituting the matrix is about 5% milligrams per square centimeter of the surface covered by the matrix, which produces a matrix about 1 mil thick on each side of the base. Good metal sponges form at about 900 C., which is below the transformation temperature of the mild steel of the base 11. Where the steel base is a mild steel strip, for example, about 5 mils thick, it may be passed through a reducing furnace of the conventional type about six feet long at a speed of about eight feet per minute, the whole length of the tube being maintained at a temperature which heatsthe strip to the recommended temperature. Under these conditions the metal of the base, particularly if it is mild steel, is practically fully annealed and nearly dead soft, and usually has a negligible spring back, such as about 18 to 20 degrees, as compared with a spring back of about 50 to 60 degrees of gas carbonized nickel coated steel strip. The spring back test is well known, and one accepted test is ,described in a publication of the American Society of Testing Materials, test B-l5 5.

Within a range of temperature of 800 C. to 1000 C. the nickel oxide is reduced without fusion to produce the nickel sponges which sinter to one another and are molecularly bound to the base by some nickel-iron alloy which forms at the interfaces of the nickel sponges and the steel base. At temperatures above the transformation point of most mild steels, which is about 930 C., some distortion of the steel strip and modification of the properties of the steel, such as ductility, may occur. At these higher temperatures the grains of nickel oxide may be reduced to solid metal particles, thus producing a matrix which is porous on account of voids between solid particles and holds a carbon coating quite well, but not as well as a matrix composed of nickel sponges. A matrix of solid particles may also be obtained by reducing the nickel oxide at a lower temperature, such as 750 C., for a much longer time, about one-half hour, which is usually not commercially feasible on account of cost. During the reduction there is some loss of volumein the layer of nickel oxide as it loses its oxygen and binder and the nickel particles sinter together resulting in the formation of a porous web-like matrix with minute pores and voids.

The matrix of sintered porous particles is rather soft and about 50% compressible, and contains no hard abrasive particles. Such a porous matrix of nickel can be deformed and burnished by a' burnisher of copper, nickel, brass, or even of soft wood, and may be compressed to a considerable extent by the pressure of the plate forming dies, so that the die wear in shaping the foundation into electrodes is slight.

A metal base surfaced witha porous matrix in accordance with my invention and without a coating is a material useful for many purposes, especially in electronic discharge devices. As the material is well annealed and well degassed, it is suitable for wires and other elements of thermionic devices, and as it holds a coating very firmly, it issuitable for coated elements, such as carbon coated grids and if the base is nickel, for oxide coated filamentary cathodes and cathode sleeves.

The carbon coating 17 may be applied to the foundation in various known ways. The foundation with a nickel base 10 may be'gas carbonized with good results. If the base 10 is a ferrous metal, such as mild steel or iron, either clear or with thin nickel plating, the carbon coating should be applied without subjecting the base to heat treatznrmee e 7 meat in the presenceof carbon. Preferably, the carbonis applied by treating the matrix with a thin suspension or carbon slurry of veryfi'nely' divided carbon; such as colloidal graphite in which as shown by photomicrographs, theparticlesrangeiinsize from about 1' micron toasmalh fraction much less than one-tenth of a micron, hence a large number-of them are smaller than the mircoscopic pores 13 in the nickel sponges. I have achieved good-results by' immersion or drag coating the' foundation with a:

kind'of carbon paint consisting of a slurry of alcohol-dag; which is a-suspension-of colloidal graphite inalcohol, and

a small amountofa' nitrocellulose binder which disappears when heated tomoderate temperatures, such as 200 C. One illustrative example of a suitable carbon slurry is about 200" grams of'al'coh'ol dag; which is 160 grams methanol and- 40 grams colloidal graphite, mixed with about 200cc: of amediunr drying binder'consisting of about 1 part by wet weight of about equal parts of /2 second and 40' second nitrocellulose and about 12% parts by weight of amyl acetate and diluted-with about 250 cc. of butyl acetate-andZOD cc. of'methanoll The matrix hasadegree and kind of porosity suchthat it absorbs the thin alcoholdagsuspension much like blotting paper absorbs writing ink. With this slurry a carbonized electrode-maybe made with agrayish black carbon'coating which is very effective. For a blacker carbon coating'to give optimumheat radiation, a mixture of 100 grams of alcohol dag'and :l gramsof lampblack may be used in place of the-200 grams of alcohol dag. Thefoundation isthoroughly wet with the slurry and dried, leaving as shown in Fig; 8the foundation covered witha thin and very firmly adherent coat 18 of carbonpaint. composed of finely divided carbon and a' smallamount-ofbinder.

The tube elements, such as plates, are made from the coated foundation, preferably by shaping them-in dies in the-usual. way. The dried-coat 18 of carbon'paint is so adherent that his not damaged in the-dies, and ingeneralno lubricantfor the dies is necessary; Where the-finished electrode is of complicated-shape, suchas shown inFig; 7, having -deep'd'rawn portions andsharp bends, some lubricationof the dies may. be advisable, inwhich case a-Iubricant such as machine oilwhich does not softennitrocellulbsemaybeusedi The coated element'is' finished and ready for use upon removal ofthe binder from-the dried coat of carbon-paint; preferably by heating in a neutral or non-oxidizing environment; The binder' disappears upon heating to a moderate temperature, such as- 200 6., leaving on the tube elementa thin, firmly adherent carbon coating. This heating of the element may be doneduringexhaust'of'the' tube in which it is-mounted, but preferably is done'prior to mountingthe tube elementin the electrode assembly.

In thepreferred practice the shaped and coated elements are generally fired atabout 800 Cl inhydrogenfor about tenminutes-topreparethem for use: This firing first-removesthe-binder from'the dried coat ofcarbonpaint and then-also cleans and finishes the element. At this temperature and under these conditions thereis negligible carburizationor-modification of'the-base by carbon. The result is a carbonized element, such as a-plate, with a carbon coating-of which the adhesion to the foundation is-greater than-its cohesion; Such a coating does not chip; peel, or flake off; If brushed, it may come-off as powdered carbon, but not in the-form of flakes orsheets', as often occurs-with the conventionalcarbonized electrodes;

The carboncoating on the finishedelectrodeis preferably as thin as feasible to produce a good black surface. For example, a successful coating hasbeen found to weigh about 2 milligrams per square centimeter of surface of the foundation.- While such a coating completely covers the matrixto an'- appreciable depth, it is very firmly bound' tozthe baseof'thematrix and' may be so'th'in that=its=surface=has practicaly the samecontour'as' the-surface of the metallic matrixof the foundation; The

g from my carbonized anode'mechanically without distorting the porousrnetal matrix. It can'be removed without substantial alteration in thestructure of thematrix by firing the carbonized anode at about l1'00 C. for about ten minutes" in hydrogen, such as'line hydrogen, which contains some" water vapor. In this way the substantially unaltered matrix- 11 of the carbonized anode can be exposed;

Carbon coated anodes; for example, made as above describedf. have been'found-by comparativemeasurements to have muchhigher thermal emittance' than similar conventionalcarbonized anodes. The thermal emittance of a carbonized anode embodying my' invention is above and usually'about'9'5% to 98% of that of a black body, whilethe thermal emittance of similar conventional commercial carbonized anodes ranges from about 60% to somewhat less"than*90% of that of a black body. These comparative measurements, made in air at 200 C. to avoid possible oxidationare" valid upto the temperatures usually attained by thea'rrodes" during tube operation.

While I' have" described the preferred embodiment of my invention iil'WBiCh ashect' of mildsteel is the base, the'matrix is ofmetalsp'onges, and the carbon coating is a' mixture of colloidal graphite and larnpblack', it is' to beunderstood thatI may'use other'metalsfor the base or heating'in'hydrogen, the'matrixmetal oxide is reduced in.

place and the reducedmetalparticles are sintered to one another and to the base, an inexpensive base metal such asirOn'Orsteelmay be used, and the carbon coating can be appliedmore' rapidly and at lower cost than by gas carboni'zing; forexa'mp'le, largely because the peculiar porosityof the matrix enables it to absorb very quickly the carbon slurry, much as blotting paper absorbs ink, thus'produci'n'g on the finished element a'thin very adherent carbon coatingwhichcompletelyicoversthe'matrix. Heretofore; ironcould notbe' satisfactorily carbonized Without a" heat treatment in the presence of carbon with unavoidable resultant carburization and impairment of the metal. By my invention'th'e ironis not impaired while beihg' carbonized; and is'in" some respects improved by the degassing an'd annealing it undergoes during the'reduction of 'the nickel' oxide.

Among the structural and other features which distinguish alcarbonized metal electrode embodying my inventions fromi the: carbonized: metal electrode of the prior arharezmbase of; practically fully annealed metal which, ifferrous metal. such as mild'z steel, is practically dead soft andhasua spring. back. of one-half or less of that of a gaswarbonizei nickel coated steel base, a porous metalr matrix of novel. texture on thebase and comprisingrparticles. of metalasintered to oneanother and molecularly'bonded' tothe base, andion the matrix an unusually adherenticoatingof. carbon'which does not peel or flake. This. carbonizedelectrode has. unusually high thermal emittance, whichmay be andusually is from 90% to 98%- ofthat of a black body. The performance in ther mionic tubes ofimycarbonized plates is so much better than thatof the conventional carbonized plates that some desirable. types 'of'tubes which. cannot be madewithconventional' carbonized plates can'be made successfully with my carbonized plates and' cannot be made commercially without them..

'Iheprefrredmethodofpracticing myinvention is a continuous process irrwhich in generala steel'strip is drag er immersion coated with nickel oxide slurry, and the coated strip passed through a reducing furnace where the nickel oxide is reduced in place on the strip, to form the matrix on the strip and thus produce the foundation which is then drag or immersion coated with the carbon paint or slurry to make the electrode stock or material from which tube elements, such as anodes may be. quickly and economically made, preferably by shaping in dies.

This continuous process will be better understood from the following more detailed description in connection with the drawing, particularly Figs. 9 and 10. In this process the base 10, in the form of a strip of mild steel, such as is known commercially as No. 1010, and for convenience about mils thick, is drawn off a supply reel 20, around pulley 23 and through an oxide slurry chamber 24 containing the nickel oxide slurry above recommended for drag coating. To apply the slurry in a thin, even coating, the strip is passed around a pulley 25 in the chamber 24 and then out of the chamber and up through an applicator 26 which, as best shown in Fig. 10, may be a pipe with two diametrically opposite slots slightly larger than the strip 10 which passes through the slots with a slight clearance. The oxide slurry in the chamber 24 is continuously circulated through the pipe by pump 27 and overflows through both slots back into the chamber 24. To obtain an oxide coating that is uniform longitudinally of the strip, the oxide particles must be kept uniformly dispersed in the suspension by the pump, and the slurry maintained at constant viscosity, which may conveniently be done by adding from a tank 28 through a needle valve 29 a regulated quantity of methanol, or other volatile agent to the contents of the chamber 24.

The oxide slurry coated strip emerges from the applicator 26 and passes vertically into the vertical drying oven 30, which may be maintained at about 200 C. and in which the volatile constituents of the binder evaporate and thus keep the nickel oxide coating and the strip so cool that the nitrocellulose does not disappear. The strip, coated with a dried coat of nickel oxide slurry, passes from the drying oven into the vertical reducing furnace 31 through a bottom slot or opening slightly larger than the strip and providing only enough clearance to preclude scraping of the dried coating off the strip. The reducing furnace has an atmosphere of hydrogen or other reducing gas supplied through an inlet 31a at slightly above atmospheric pressure, and is run at a temperature which keeps the strip in the furnace at the chosen temperature within the range of 800 C. to 1100 0, thereby causing decomposition of the nitrocellulose and reduction of the nickel oxide. At the chosen temperature the nickel particles resulting from the reduction are well sintered to one another and molecularly bonded to the steel strip. The temperature of the strip in the furnace and its speed are such that while good adherence of the reduced nickel to the steel is achieved by incipient fusion of nickel iron alloy at the interface, the bulk of the reduced nickel is in the form of a porous matrix of microscopically porous nickel sponges. The furnace 31 is a usual type, about six feet long, and is maintained at about the same temperature from end to end. The strip may be passed through the furnace at a speed of about eight feet per minute. The cool strip entering the hot furnace at this speed is subjected to a rather great and sudden increase of temperature. This sharp temperature gradient is advantageous in producing the desired structure of the nickel matrix. A by-product of the reduction is water vapor which tends to have a decarburizing and softening action on the steel.

The strip 10 emerges from the furnace 31 as a foundation with a matrix coating of sintered metallic nickel which is soft, highly porous, and bonded to the metal strip. The extreme porosity can be readily shown by marking with ink, which is absorbed almost completely as by blotting paper. With the prior strips with which I am familiar, ink marks dry on the surface and remain permanently visible as if on paper. Upon leaving the furnace the nickel surfaced strip passes through a cooling chamber 32, around pulley 33 enclosed in a gas tight housing, and down through another cooling chamber 34, from which it emerges through a bottom slot and at substantially room temperature. Both cooling chambers and the housing for the pulley 33 are kept filled with hydrogen from the furnace, as the cooling chamber 32 opens into the furnace and also into the housing around the pulley 33.

As the foundation strip emerges from the cooling chamber 34, it is ready for the application of a coating, such as carbon. It may be stored, if desired, and the coating applied later as a separate operation. I prefer to pass the foundation strip directly on down into a carbon applicator chamber 35, around pulley 36 in the chamber, and up through another applicator 37, similar to applicator 26, and through which flows the carbon slurry, main-' tained at proper viscosity by a small methanol tank 38 for supplying methanol through a needle valve 39, and circulated through the applicator 37 by a pump 40. The foundation strip, with a uniform coat of carbon paint or slurry which has entered the pores and crevices of the nickel matrix much as ink enters blotting paper, nextenters drying oven 41, similar to the oven 30 and maintained at about 200 C. to produce a dried coat 18 of carbon paint on and covering the matrix. The binder in the carbon slurry binds the carbon particles together on the surface, in the pores, and in crevices of the nickel matrix. The carbon may be either amorphous or graphitic, depending upon the nature of the carbon added to the suspension in applicator chamber 35. The foundation strip with a dried coat of carbon paint 18, as shown in Fig. 8, passes out of the oven 41 and around pulley 42 to a reel, not shown, and then may be stored as electrode stock from which electrodes and tube elements may be made. The carbon can in this way be applied to the strip with the matrix surface much more rapidly than by gas carbonizing methods, in which the carbon is applied by deposition from hydrocarbon gases. A speed of eight feet per minute, which is ten times as rapid as the gas carbonizing process, has been obtained in my method.

With a 5 mil steel strip the coating of nickel matrix and carbon paint on both surfaces raises the thickness of the foundation strip at the most to only about 7.5 mils. No difliculty will therefore be encountered in making vacuum tube plates with dies designed for 5 mil strip, as the foundation strip made by my method with a coat of carbon paint on it is readily compressed in the dies. However, if desired, this foundation strip may be run through rolls and compressed. I have in this way reduced the coating on each surface of the strip to a thickness of less than 1 mil without materially affecting the heat radiating characteristics of the finished electrode. From metal coated by my improved method, compressed or not, carbonized electrodes may be made that have heat radiation power or thermal emittance more nearly approaching that of a black body than metal carbonized by any other method known to me.

An additional advantage of my method is that the steel strip as supplied by the manufacturer can be stored and then used without cleaning, as rust on the steel strip is reduced in the reducing chamber and oil film on the strip can also be tolerated.

The coat of carbon paint on the nickel matrix coated strip adheres very firmly to the nickel and does not pack in the dies when the tube parts are formed. The tube parts made therefrom may be fired in hydrogen at about 800 C. for 10 minutes, as customary, without depletion of the carbon. Another advantage of the electrode or plate stock made according to my invention is that the comparatively rigorous firing tends to degas and purify the material.

My method of carbon coating metal may be employed amasse- 11 with various metais. It will be found" quite suitable for coating nickel strips as well= as ferrous metal strips. also canbe'us'ed to obtain metal surfaces oftwo' different rnetals' siutered together and composed of metals that cannot be applied to each" other by electrolytic plating or by any other method at all. For example; nickel and copper which will not co=deposit electrolytically are easily applied together. It is also possible to include non-metallic substances with the metals by mixingthem in the slurry. Various other modifications may be'm'ad'e in. the method" and product without departing from the spirit of the invention.

What is claimed is:'

1. A carbonized. m'etal' article comprising'a sheet metal base, and a spongy rough surface bonding layer l'tonde'd to said'base, saidibonding layer'having' a coating thereon of colloidal" graphite, said bonding layer" consisting en'- tirely of individually microscopically porous nickel particles sintered' to one another; wherebyenipty spaces are provided' in said bonding layer in the form of pores in the individual nickel particles and voids between the sintered nickel'parti'cles, the particles in. contact with'sa'i'd' base being sint'ered to the base for bonding said' layerrto the base; the outer surface of said' bonding layer consisting of'a layer ofsaid particles,saidparticles'in said layer defining voids therebet'ween andliaving pores there'- in communicating with said surface, said coating ofcol loidal graphite being in intimate contact with the surfaces ofsaid particles in said layer definingsaid voids and the pores therein, wherebysaid' coating. is strongly adherent to said article to permit working of the article without loss of the coating and said' coating, is characterized by improved continuity on said outer surface for causingwsaid article to have improved heat radiating properties.

2. A continuous methodof making a metal strip having a coating of carbon thereon wherein the coating is characterizedby improved adherence to assure freedom from loss.duringasubsequent working of'the coatedstrips; said method comprisingthe steps of coating said metal strip with a non-compacted layerof. finely powdered nickel oxide in a binder evanescent. at. a temperature below 900 C.; passing the coated'strip through a furnace havinga reducing atmosphere. and a temperature. of about 900 Cato reduce said nickel oxide to porous particles of nickel in situ on said strip, and to simultaneously I2 siiitersaiid po'ro'u snickel particles to one another into a spongy layer and bondsaid layer to said strip, said temperature' being"belbw the temperature at which said' orous particles collapse; passing" said strip through a carbon slurryof very finely divided carbonsuspended in ath'in liquidbinder, whereby said slurry is absorbed by said spongy layer and covers said spongy layer with a continuous coating of carbon, and drying said coating; whereby saidcoating'is'strongly adherent to said matrix and said coated metal strip is" characterized by'imp'roved' heat radiation, for advantageous use" in an electron disclra'rge device.

3". A metal element com rising a structure having thereona coating ofca'rb'on' forirrrprovedheat dissipation andbeiiig adapted to be worked and formed for use in an electron discharge device without a reciable loss of saidc'oating, said structurecomprising a non-porous base made of a metal selected from the group consisting of mild steel and'nickel; and a nickel matrix bonding layer bonded to said base, said bonding layer consisting entirely ofnick'el particles individually having pores therein and'sint'e'red' to each other, said bonding layer having an-outer surface consistingtof some of said particles, said some of said particles having: voids therebetween and pores therein-communicating with said'surf'ace, said coatingfilling said'voids and pores, whereby said'element has a continuous carbon coating characterized by increased heat dissipation and is adapted to be- Worked and formed with reduced loss of coating material:

References Citedin thefileofthis patent UNITED' STATES PATENTS 1,197,694 Watkins- Sept. 12, 1916 1 ,703 ;l7 7- Short Feb. 26, 1929 1,922,254 McCullough Aug. 15, 1933 2,133,761 Tietig Oct, 18, 1938 2,147,447 Kolligs Feb. 14, 1939' 2,187,086 Koehring Ian: 16, 1940 2,190,237 Koeliring Feb. 13, 1940 2,239,4'l 4 Eddison Apr; 22, 1941 2,2 s9 i6l 4 Wesley July- 14, 1942 FOREIGN PATENTS 965342 1 France Sept. 12, 1950 

1. A CARBONIZED METAL ARTICLE COMPRISING A SHEET METAL BASE, AND A SPONG ROUGH SURFACE BONDING LAYER BONDED TO SAID BASE, SAID BONDING LAYER HAVING A COATING THEREON OF COLLOIDAL GRAPHITE, SAID BONDING LAYER CONSISTING ENTIRELY OF INDIVIDUALLY MICROSCOPICALLY POROUS NICKEL PARTICLES SINTERED TO ONE ANOTHER , WHEREBY EMPTY SPACES ARE PROVIDED IN SAID BONDING LAYER IN THE FORM OF PORES IN THE INDIVIDUAL NICKEL PARTICLES AND VOIDS BETWEEN HE SINTERED NICKEL PARTICLES, THE PARTICLES IN CONTACT WITH SAID BASE BEING SINTERED TO THE BASE FOR BONDING SAID LAYER TO THE BASE, THE OUTER SURFACE OF SAID BONDING LAYER CONSISTING OF A LAYER OF SAID PARTICLES, SAID PARTICLES IN SAID LAYER DEFINING VOIDS THEREBETWEEN AND HAVING PORES THEREIN COMMUNICATING WITH SAID SURFACE, SAID COATING OF COLLOIDAL GRAPHITE BEING IN INTIMATE CONTACT WITH THE SURFACES OF SAID PARTICLES IN SAID LAYER DEFINING SAID VOIDS AND THE PORES THEREIN, WHEREBY SAID COATING IS STRONGLY ADHERENT TO SAID ARTICLE TO PERMIT WORKING OF THE ARTICLE WITHOUT LOSS OF THE COATING AND SAID COATING IS CHARACTERIZED BY IMPROVED CONTINUITY OF SAID OUTER SURFACE FOR CAUSING SAID ARTICLE TO HAVE IMPROVED HEAT RADIATING PROPERTIES. 